Austenitic alloy



United States Patent 3,175,902 AUSTENKTIC ALLOY Joseph A. Feiree, in, Natrona Heights, Pa, assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa, a corporation of Pennsylvania No Drawing. Filed Nov. 6, 1962, Ser. No. 235,867 6 Claims. (til. 75128) This invention relates to fully austenitic stainless steel and in particular to fully austenitic stainless steels which are characterized by having a constant density within given limits and which are particularly suited for use as mass standards.

At the present time, there is no standard weight material. Brass which has often been employed has numerous disadvantages in that it is not resistant to corrosion and, consequently, may show significant variations in mass with time because of oxidation and/or corrosion. In addition, brass has a low resistance to abrasion and may lose some weight in use. In order to alleviate these conditions, resort has been sometimes had to gold plating of the brass mass standards in order to overcome the corrosion and oxidation problems. However, gold plate also has a low abrasion resistance and in some instances, the gold plate may exhibit surface imperfections which may cause accelerated corrosive attack by galvanic action with the base metal.

Variations in density due to difierences in composition, unsoundness, dissolved gases and non-metallic inclusions are other sources of inaccuracies. Moreover, differences in density readily complicate calibration of mass standards because the buoyancy effects of the atmosphere will be different for standards with the same mass but different volumes. In order to attain some uniformity for mass standards the National Bureau of Standards has proposed a number of requirements which must be met for a material to be used as an optimum mass standard. These requirements include:

(1) Sufiicient abrasion resistance to prevent loss in mass during regular, careful usage.

(2) The material must have the ability to be polished to a mirror finish.

(3) The material must be non-magnetic.

(4) The material must be resistant to atmospheric corrosion and oxidation to avoid the necessity for plating; and

(5) The material must have a constant uniform density of 8.0101 g./ml. at 25 C.

The fully austenitic steel of the present invention meets the requirements set forth by the National Bureau of Standards.

An object of this invention is to provide a fully austenitic stainless steel suitable for use as a mass standard.

Another objeciof this invention is to provide a fully austenitic steel having a preferred balanced composition within given limits to provide a density within the range between 8.0i0.1 g./ml. at 25 C.

A more specific object of this invention is to provide an iron-chromium-nickel-molybdenum fully austenitic stainless steel having good abrasion resistance and a balanced composition to provide a constant uniform density of 8.0i0.l g./ml. at 25 C.

Other objects of this invention will become apparent to those skilled in the art when taken in conjunction with the following description.

The steel of the present invention in its broader aspects contemplates a composition which includes up to 0.10% carbon maximum, from 1.0% to 20% manganese, up to 0.03% maximum phosphorus, up to 0.03% maximum sulfur, from about 0.25% to about 1.0% silicon, from about 17.5% to about 22.5% chromium, from about aussaz Fatented Mar. 30, 19

23.0% to about 27.5% nickel, from about 1.0% to about 3.25% molybdenum, optional to about 1.5% maximum tungsten, optional to about 0.5% maximum copper and the balance substantially iron with incidental impurities.

Each of the elements present within the composition set forth Within the broad ranges enumerated hereinbefore performs a specific function. While carbon is present up to a maximum of 0.10%, it is preferred to maintain the carbon content as low as possible Within reasonably commerical melting practices. This results from the fact that it is desirable to afford the steel with a maximum amount of corrosion resistance and the utilization of low amounts of carbon are efiective for providing the maximum amount of chromium for optimum corrosion resistance.

Manganese is present for conferring hot workability to the steel of the present invention and in this respect, it should be noted that manganese will also combine with certain gases to thereby reduce the amount thereof thus removing one of the inaccuracies resulting in variations in density. It is preferred to have at least 1.0% manganese present and the addition of more than about 2.0% does not appear to be beneficial. Optimum results have been obtained where the manganese content is maintained within the range between about 1.25% and about 1.75%.

Silicon is preferably present within the range between 0.25% and about 1.0% and acts in conjunction with the manganese in aiding the fabrication of the steel and in removing the amount of occluded gases. Preferably the silicon content does not exceed about 1.0%. Optimum results have been obtained where the silicon content is maintained between the range of 0.4% and 0.6%.

Chromium is present within the steel of the present invention in a minimum amount of 17.5% in order to obtain the required degree of corrosion resistance. White chromium can be present up to about 22.5%, it is preferred not to exceed this upper limit in order to insure freedom from the presence of delta ferrite within the microstructure thus insuring that the steel will have a minimum magnetic permeability. Where the chromium content is maintained within the optimum range of about 19.0% to about 21.0%, the steel is afforded a combination of a maximum degree of corrosion resistance consistent with constant magnetic permeability and a control of the density to within $0.05 g./ml. from the ideal density of about 8.0 g./ml. where the balance of the alloying components are proportioned as will be described hereinafter.

The steel of the present invention contains between 23.0% and 27.5% nickel, which functions to insure a fully austenitic structure which is characterized by having an austenite stability which will prevent any possible transformation of the non-magnetic austenite to magnetic martensite during such operations as machining, polishing and subsequent usage. The optimum austenite stability is obtained where the nickel content is maintained within the range between 24.0% and about 26.0%. Moreover, nickel together with chromium aids the corrosion resistance.

The steel of the present invention utilizes molybdenum in the minimum amout of 1.0% whereas the maximum should not exceed about 3.25%. The element molybdenum confers a high degree of corrosion resistance especially where the atmosphere may contain one of the halogen elements. Molybdenum also contributes to the abrasion resistance of the steel and is particularly useful for balancing the composition with respect to obtaining the required density since the element molybdenum has a relatively high density. in this respect, it should be noted that it is possible to substitute up to approximately 1.5% maximum tungsten for the molybdenum. While tungstem is also effective for providing the steel with an increased measure of resistance to abrasion, none the less, it does not provide the additional corrosion resistance which is obtainable through the use of molybdenum. Accordirical formula specifically for the range of analysis set forth in Table I which will assure a density within the limits of 8.0:01 g./ml. at 25 C. This empirical formula enables the adjustment of the chemistry of the steel ingly, it is preferred that the steel of the present invention 5 prior to tapping and will insure the final production to utilize molybdenum in preference to tungsten. Optimum have the required density. The empirical formula 15 as results appear to be obtained where the molybdenum follows:

content is maintained within the range between about mula is as follows:

and about 275% Density=0.0014l5 [(%Fe) (55.85)+(%Mn) 54.93

Copper may also be presentwithin the steel of the (2806)+(%Cr) (5201)+(%Ni) present invention; however, it is preferred to limit the (58 69)+(%Mo) 1 392 copper content to about 0.5% maximum. Normal nielt- +(%cu) (6354)] ing operations are usually effective for obtaining low copper contents and preferably the copper should not be It to be noted a e empirical formula Set forth in excess f b t 0,2% 15 does not contain any provisions for such elements as The balance of the steel is substantially iron with the eflrbml, nitrogen, Phosphorus and Shtfhh Smee these normalamounts of phosphorus and sulfur which it is preelemenfis are irly low and are als fairly Constant their ferred to maintain at the minimum. In this same respect, effect On the d nsity is negligible. it is also desired to maintain the other incidental ime e the etteet h of the etethjchts t purities such as dissolved oxygen and nitrogen at a minigehese slheoh t chrohhhht which have atohhc Welghts mum in order to obtain the highest quality of material. than that of H011 are ll'lvefsely Propomonal to the As stated hereinbefore, freedom from dissolved gases, Welght Pereehtage- Thus W the Perchhteges of these unsoundness and non-metallic inclusions is required in e e e the de'hstty e Yet the total ethect order to minimize the amount of inaccuracies and varia- 1S St1h adthttve and must hetohstdered ht Conjunction tions in density. Consequently, it is preferred to vacuum Wlth the balance of h w etethehts there Preseht' melt Th6 stfiel of the pressnt invention This. may be Consequently, when slight variations in the ad ustment of conveniently accomplished through the utilization of vacthe hhal eherhlstry necessarr must be kept n mind uum induction melting and further refinements of the steel that the adthtthh of manganese Stheoh and chrothhtht W111 are possible by utilizing the vacuum melted material as a reduce the eatchtated dehstty Whereas the adthttoh 0t consumable electrode for the production of consumable e elements nickel, molybdenum: tungsten copper electrode vacuum remelt material. By thus utilizing the lhefeases the ly- Thus, fr m th foregoing emforegoing melting techniques which are common in the P t h h Whtch ,relatee only to t Steel of thts art, it is possible to minimize the inaccuracies due to nonthvehttoh Wtthth e hthtts,set forth hetethhetore the hhal metallic inclusions, dissolved gases and unsoundess of the dehstty be Prethcted t the hhhts Set forth through finally producgd product It should be noted, however, the balance of the chemical composition of each of the that the melting practice per se is not a limitation on the ahoythg componentssteel of the present invention and Where desired other In other m clearly hehtohsttate the steel of the melting practices may be employed in order t Obtain the present invent on, reference is directed to Table II which steel of the presentinvenfiom contains the identity of three heats which were made Reference is respectfully directed to Table I Whichi11u6 40 and tested following the teachings of the present invention. trates the chemical composition of the steel of the present These e h the composttthh Set forth in Table H invention it being noted that Table I Contains the limits and the utilization of the empirical formula set forth for thg general range as Well as the prefenfid range of hereinbefore forms the basis for the recorded data for the composition. theoretical density. The actual density measurements in TABLE I g./ml. at 25 C. are also set forth showing the close agreement between theoretical and actual density. Chemical composltzon [Percent by weight] TABLE II Element (ipgigggl Pfigrgd Element Heat X Heat 0 Heat N O .092 .003 0007 Mn 1.44 1.40 1.46 Si 0. 54 0. 53 0. 40 P 0. 024, 0.021 0.010 s. 0. 01s 0. 021 0. 013 55 Cr 20.02 10. 84 20. 20 Ni 24. 88 25. 05 25. 54 Mo. 2. 07 2. 05 2.38 W 0.13 ou 0.20 0.10 0. 20 N 0.024 0.030 Fe 50. 68 50.58 49.68 Theoretical De 7.98 8.0 7.99 I Actual Density 7 98 8.0 8.01

1 Optional to stated limit with a corresponding percent decrease in molybdenum.

As as S d hfireinbefefe, the Steel 051116 P e Steel from the foregoing heats was readily fabricated yentwn s Partwular usage as a constant d n y alloy into bars by hot rolling, there being no difficulty encounin the fabrication of mass standards. Th se Standards 5 tered. Thereafter pieces from the bars were machined, requirethat the steel have a constant uniform density of di d b i h d b th th d d ib d b E, H 8.0i 0.1 g./ml at 25 C. However, in order to be com- Hull Machinery, January 1962, pages 92 through 97, mercially feasible, it was necessary to devise a procedure which is in accordance with the requirements set forth by for predicting the final dens ty of a melt prior to tapping the National Bureau of Standards, and polished with 600 while adjustments in the analysis can be made. This 70 grit silicon carbide paper for testing. Measurement of 7 resulted from the fact that certain combinations of the the actual density as recorded in Table 11 compared exalloying elements within the range set forth hereinbefore ceed'ingly well with the theoretical density calculations in Table I would not produce densities within the remade by utilizing the empirical formula set forth hereinquired limits. before. Thus, with the close control and agreement of Experimental procedures resulted in deriving an emthe actual density with the theoretical density, the initial requirement of the material having the controlled density is adequately met. Accordingly, the empirical formula set forth hereinbefore becomes a practical means for controlling the chemistry of the composition in order to obtain a steel having a predetermined density.

Additional samples from Heat N after diamond burnishing were measured (for surface roughness. As diamond burnished, Heat N exhibited a profilimeter reading of 5.6 R.M.S. microinches. Thereafter, the bar material was polished with 600 grit silicon carbide paper, and again using a profilimeter, the surface roughness measurements exhibited by the bar decreased to 0.8 R.M.S. microinch. The National Bureau of Standards has indicated that a surface roughness of 1.2 R.M.S. microinches is quite satisfactory. As indicated, the steel of the present invention easily meets this capability. Further tests were run on Heat N, which included surface hardness. These test results, converted into a Brinell Hardness Number from the Rockwell -T, demonstrated a hardness equivalent to 185 BHN. This hardness was for material in the annealed condition after diamond burnishing and polishing. Contrasted thereto, cold worked material exhibited a hardness of over 200 BHN, depending upon the amount of cold work. By comparison, annealed gold exhibits a hardness of about 25 BHN, and brass about 60 BHN, thus indicating the outstanding hardness and abrasion resistance exhibited by the steel of the present invention in comparison with both gold and brass which have been heretofore used as constant mass standards.

Heat N was also subjected to a measurement of its magnetic permeability in the annealed condition, as well as in the cold worked condition in order to determine the stability of the austenite. These measurements indicated that Heat N exhibited a magnetic permeability of less than 1.005 at 200H. After heavy cold reductions, no magnetic phases were detected. Since Magna gauge tests on the surface of the polished bar indicated a permeability of less than 1.005, which is the lowest limit as respects the sensitivity of the Magna gauge in measuring the magnetic permeability, it is clear that the present alloy possesses a high degree of austenite stability.

The most stringent characteristic of a constant mass standard relates to its corrosion resistance. Material from Heat N was also subjected to the standard corrosion resistance test which simulates conditions to be normally found in the careful usage of such a material as a constant mass standard. Samples from Heat N were cleaned with nitric acid at room temperature to remove any contaminants which may have been present after burnishing and polishing. Samples were exposed to 100% relative humidity at a temperature within the range of from 100 to 125 F. After four days exposure there was no corrosive attack or discoloration noted. These same samples were also exposed to 100% relative humidity at room temperature for three days without any detectable changes having occurred. Thereafter these same samples were tested in 5% neutral salt spray for a period of eight days. After eight days the material was again exam ined, and there was no visible corrosive attack, discoloration or any other adverse effect noted from this test.

No special skills are required in producing this alloy, and regular commercial mill equipment was utilized in the production thereof.

I claim:

1. A constant density alloy consisting essentially of up to 0.10% carbon, from 1.0% to 2.0% manganese, from 0.25% to 1.0% silicon, from 17.5% to 22.5 chromium, from 23.0% to 27.5% nickel, from 1.0% to 3.25% molybdenum and the balance essentially iron with incicental impurities and which is characterized by exhibiting 6 a constant density within the range between 7.9 and 8.1 g./ml. at 25 C.

2. A constant density alloy consisting essentially of up to 0.03% carbon, from 1.25% to 1.75% manganese, from 0.40% to 0.60% silicon, from 19.0% to 21.0% chromium, from 24.0% to 26.0% nickel, from 1.25 to 2.75% molybdenum and the balance essentially iron with incidental impurities and which is characterized by exhibiting a constant density within the range between 7.95 and 8.05 g./m l. at 25 C.

3. A fully austenitic stainless steel particularly suited for use as a constant mass standard consisting essentially of up to 0.10% carbon, from 1.0% to 2.0% manganese, from 0.25% to 1.0% silicon, from 17.5% to 22.5 chromium, from 23.0% to 27.5 nickel, from 1.0% to 3.25% molybdenum, optional to 1.5 tungsten, optional to 0.5% copper and the balance essentially iron with incidental impurities, and which is characterized by exhibiting a density 5 within the range between 7.9 and 8.1 g./ml. at 25 C. resulting from balancing the constituents of the alloy within the ranges stated according to the following formula:

4. A fully austenitic stainless steel particularly suited for use as a constant mass standard consisting essentially of up to 0.03% carbon, from 1.25% to 1.75% manganese, from 0.40% to 0.60% silicon, from 19.0% to 21.0% chromium, from 24.0% to 26.0% nickel, from 1.25% to 2.95% molybdenum, optional to 1.25% tungsten, optional to 0.2% copper and the balance essentially iron with incidental impurities, and which is characterized by exhibiting a density 6 within the range between 7.95 and 8.05 g./ml. at 25 C. resulting from balancing the constituents of the alloy within the ranges stated according to the following formula:

5. A constant mass standard formed from a fully austenitic stainless steel having a composition consisting essentially of up to 0.10% carbon, from 1.0% to 2.0% manganese, from 0.25 to 1.0% silicon, from 17.5 to 22.5% chromium, from 23.0% to 27.5 nickel, from 1.0% to 3.25% molybdenum, optional to 1.5% tungsten, up to 0.5% copper, and the balance essentially iron with incidental impurities, characterized in that each of the constituents of the metal is balanced to provide the mass standard with a density within the range between 7.9 and 8.1 g./ml. at 25 C.

6. A constant mass standard formed from a fully austenitic stainless steel having a composition consisting essentially of up to 0.03% carbon, from 1.25% to 1.75 manganese, from 0.4% to 0.6% silicon, from 19.0% to 21% chromium, from 24% to 26% nickel, from 1.25% to 2.75% molybdenum, optional to 1.25% tungsten, up

to 0.2% copper, and the balance essentially iron with incidental impurities, characterized in that each of the constituents of the metal is balanced to provide the mass standard with a density within the range between 7.95 and 8.05 g./ml. at 25 C.

References Cited by the Examiner UNITED STATES PATENTS 2,606,113 8/52 Payson et a1. -1289 DAVID L. RECK, Primary Examiner. 

1. A CONSTANT DENSITY ALLOY CONSISTING ESSENTAILLY OF UP TO 0.10% CARBAON, FROM 1.0% TO 2.0% MANGANESE, FROM 0.2K% TO 1.0% SILICON, FROM 17.5% TO 22.5% CHROMIUN, FROM 23.0% TO 27.5% NICKEL, FROM 1.0% TO 3.25% MOLYBDENUM AND THE BALANCE ESSENTAILLY IRON WITH INCIDENTAL IMPURITIES AND WHICH IS CHARACTERIZED BY EXHIBITING A CONSTANT DENSITY WITHIN THE RANGE BETWEN 7.9 AND 8.1 G./ML. AT 25*C. 